One-time Sandinista revolutionary Daniel Ortega took a big early lead in presidential elections on Sunday, amid reports of protests and international observers being blocked from voting stations.

Ortega, the incumbent and favourite, had 66% of the votes compared to 25% for his nearest challenger, Fabio Gadea. Conservative Arnoldo Alemán, a former president, was a distant third with 7%.

The result came with roughly 7% of the votes counted, but the electoral council president, Roberto Rivas, said a quick count representative of the entire vote gave Ortega a large advantage as well. The methodology of the quick count was not immediately available, however.

The ruling Sandinista party declared victory and caravans of thousands of supporters flooded the streets shouting: "Daniel! Daniel!"

International observers reported problems with access to voting stations and with one national group of observers, Let's Have Democracy, reporting 600 complaints of voting irregularities, a handful of injuries in protests and 30 arrests.

The head of the Organisation of American States (OAS) observer mission, Dante Caputo, said its staff were been denied access to 10 polling stations, which would account for 20% of the statistical material they had planned to collect for their analysis. "They have prevented our people from being there at the precise moment they should have been there and that is not remediable and will affect our ability to do our jobs," he said. "We are navigating without radar."

The EU said some of its teams also had problems but they eventually were resolved and allowed access, according Luis Yanis, head of the mission.

The Ortega government, meanwhile, reported smooth voting in 90% of the country.

"We have seen a civil day of voting," Rivas said during a news conference, emphasising the lack of violence and calling it the cleanest Nicaraguan election he had observed in recent years.

Gadea, who went into election day trailing Ortega in the polls by 18 points, thanked voters in a brief press conference for coming out en masse. "The attempt to discourage voting and create difficulties has failed," said Gadea of the Liberal Independent party. "No one or nothing will alter the will of the people."

Since returning to power in 2007, 65-year-old Ortega has boosted his popularity in Central America's poorest country with a combination of pork-barrel populism and support for the free-market economy he once opposed. He seeks a third term – his second consecutive one – after the Sandinista majority on the supreme court overruled the term limits set by the Nicaraguan constitution. His opponents fear that if he wins more than 50% of the vote, it will allow him to change the constitution to run in perpetuity.

The independent Let's Have Democracy reported problems in various municipalities located between 50 and 70 miles outside of the capital of Managua. Besides injuries and arrests, observers reported a polling booth set on fire, election officials obstructing voters from opposing parties and protests by voters who didn't receive their credentials.

The sporadic incidents did not seem widespread enough to throw the official results into question. But it was unclear whether the OAS would question the results because of lack of access to polling stations.

Claims of widespread fraud in the 2008 municipal elections led Washington to cancel $62m (£39m) in development aid. Nicaragua's 2006 election drew more than 18,000 observers. This time election observation is more difficult and local observers were denied credentials. The OAS and the EU negotiated access to Sunday's vote, but the Georgia-based Carter Centre decided not to observe because of the restrictions.

Ortega led the Sandinista movement that overthrew dictator Anastasio Somoza in 1979, and withstood a concerted effort by the US government, which viewed him as a Soviet-backed threat, to oust him through a rebel force called the Contras.

The fiery leftist ruled through a junta, then was elected in 1984 but was defeated after one term in 1990. After two more failed runs, he softened his rhetoric, took a free-market stance, and regained the presidency in the 2006 election.

While the left seemed to be heading for victory in Nicaragua on Sunday, a rightwing former general promising to get tough on crime won presidential elections in Guatemala.

Otto Pérez Molina of the conservative Patriotic party won 55% of the vote, beating the tycoon-turned-political populist Manuel Baldizón of the Democratic Freedom Revival party, who had 45%.

Pérez, 61, is the first former military leader elected president in Guatemala in the 25 years since the end of brutal military rule. While that concerns some international groups, Guatemala has a young population, and many do not remember the war.

Witnesses say hundreds of villages were obliterated by the army's scorched-earth policy. Pérez has said there were no massacres or genocide. He has never been charged with any atrocities and was one of the army's chief representatives in negotiating the 1996 peace accords.

The outgoing centre-left president Álvaro Colom, who cannot run for re-election, urged both sides to respect the results.

More than half of Guatemalans live in poverty exacerbated by organised crime and Mexican drug cartels. The country, which has a population of 14 million, has one of the highest murder rates in the world, a product of gang and cartel violence, along with the legacy of its 1960-1996 civil war in which the army, police and paramilitary are blamed for killing the vast majority of 200,000 victims – most of whom were Mayan.